1. Field of the Invention
The present invention relates to electrical overcurrent circuitry for protecting electrical equipment, such as electrical motors and the like, and more particularly, to electrical overcurrent circuitry for protecting conductors within such equipment from damage due to overheating which includes digital and analog modeling of the conductor temperature which enables the circuitry to simulate a conductor cooling condition following a trip of the electrical equipment or loss of electrical power to the electrical overcurrent circuitry.
2. Description of the Prior Art
Various devices are known in the art for protecting electrical equipment, such as electrical motors and the like, from damage due to conductor overheating. Examples of such devices include circuit breakers, overload relays, motor protectors, and the like. Such devices are intended to limit the temperature rise of the conductors within the electrical equipment (e.g., rotor and stator windings of an electrical motor) to prevent damage due to overheating. More specifically, current based time curves, known as overload protection curves, are normally provided by electrical equipment manufacturers which provide the maximum tripping times as a function of the electrical current that the conductors can withstand without exceeding the maximum temperature rise of the conductors and thereby damaging the electrical equipment.
Overcurrent devices are normally provided with a tripping characteristic adapted to trip the electrical equipment before the maximum temperature rise is exceeded. The tripping characteristics of the various overcurrent devices are provided by various mechanical and electrical means. For example, some known overcurrent devices, such as circuit breakers, incorporate bimetals disposed in series with the line conductors feeding the equipment which disconnect the electrical equipment from the source of electrical power in accordance with predetermined heating characteristics of the bimetals.
Other known overcurrent devices utilize electronic tripping and may incorporate a microprocessor to generate a current based time function based on the maximum temperature rise of the conductors in the electrical equipment to be protected. Such devices normally include sensors for sensing the electrical current to the electrical equipment in order to approximate the conductor temperature. In such devices for a given overcurrent value, known as a pick up value, a timing function is initiated. If the overcurrent condition persists for a sufficient period of time, the electrical equipment is tripped. If, however, the current drops below the pick up level during the time out period, the timer is generally decreased to a value corresponding to the value at the pick up level to simulate a conductor cooling condition. This allows the overcurrent device to provide protection based on the actual conductor temperature. Should the power supply to the electrical overcurrent device be lost following a trip of the electrical equipment, all timing functions executed by the microprocessor are generally lost.
Analog timing circuits including a capacitor have been used to keep track of the time that the electrical power supply has been lost. More specifically, in such known devices, every time a trip is initiated a trip flag is set by the microprocessor and stored. While power is available, the capacitor is charged to a fixed value. On power up of such an overcurrent device, the trip flag is normally checked to determine if a trip occurred. If so, the voltage across the capacitor is sensed and used to calculate the elapsed time that the power supply to the overcurrent device was lost. Once the elapsed time is calculated, the microprocessor then calculates the cool down temperature in order to adjust its timing function to correspond to a conductor cooling condition.
Under certain conditions, the power supply to the electrical overcurrent device may be lost without a corresponding trip of the electrical equipment. Such conditions may include a trip of an upstream circuit breaker in a radial feed system. In other known applications, the overcurrent circuitry is powered by the circuit breaker being protected. Thus, in such applications, the power supply to the overcurrent circuitry is lost when the circuit breaker is tripped. Under such conditions, the circuitry heretofore described would be unable to simulate a conductor cooling condition which could result in damage to the electrical equipment to be protected.